1. Field of the Invention
This invention relates to a chemical amplification photoresist for use in a photolithography step in the production of a semiconductor device such as an LSI and more particularly to a positive-type chemical amplification photoresist which becomes soluble in a basic developer upon exposure to light.
This application is based on Japanese Patent Application No. Hei 10-171558, the content of which is incorporated herein by reference.
2. Description of Related Art
In photolithography using chemical amplification photoresist for the production of semiconductor devices, the g-line (wavelength: 436 nm) and i-line (wavelength: 365 nm) of high pressure mercury lamps have heretofore been used as an exposure light source together with a novolak type resin as a photoresist for forming a resist mask on a semiconductor substrate.
However, with the development of more highly integrated LSI devices, exposure utilizing far ultraviolet rays such as excimer laser beams (wavelengths: 248 nm, 193 nm), which is more advantageous than exposure with the g-line or i-line as a light source, has been considered more desirable.
On the other hand, the use of a KrF excimer laser as a light source causes the problem that novolak photoresists, which strongly absorb far ultraviolet rays, produce no acceptable resist pattern.
Accordingly, in order to solve this problem, a chemical amplification photoresist as described in Japanese Patent Application, Second Publication (Kokoku) No. Hei 2-27660, has been proposed. The chemical amplification photoresist is a positive-type photoresist utilizing an acid-catalyzed reaction, comprising a base resin, such as polyhydroxystyrene, which is alkali-insoluble in the state in which a protecting group is attached to a predetermined site of the base resin but is alkali-soluble in the state in which the protecting group is eliminated therefrom, a photochemical acid generator which generates hydrogen ions upon exposure to light, a small amount of additive for adjusting the performance, and an organic solvent used as a diluent for spinner coating.
After coating the chemical amplification photoresist on a semiconductor substrate and drying and solidifying it, irradiation of far ultraviolet rays from an excimer laser light source to the resulting photoresist film on the semiconductor substrate results in generation of hydrogen ions (acid), which is an initiator species for chemical amplification, by the photochemical acid generator. The hydrogen ion substitutes for a protecting group attached to the base resin to release the protecting group in the process of heat treatment (PEB: Post Exposure Bake) carried out subsequently to the exposure, thereby changing the photoresist from alkali-insoluble to alkali-soluble, with concomitant generation of a by-produced hydrogen ion, which allows the reaction of eliminating the protecting groups from the base resin to proceed like a chain reaction. This reaction, which is called an acid-catalyzed sensitization reaction, increases the dissolution selectivity of photoresist so that highly sensitive photosensitive properties can be achieved. Therefore, development of the photoresist after exposure with an alkaline developer results in a desired fine resist pattern. (cf. Hattori et al., "Proceedings of SPIE" Mar. 1, 1993 p.149 Line 33 Schemel & Hattori et al., "Journal of Photopolymer Science and Technology" Jun. 23, 1993 p. 503 Line 2 Scheme 1).
In the initial stage of development of the positive-type chemical amplification photoresist, a tert-butoxycarbonyl group represented by the following Chemical Formula 1 was used as a protecting group. ##STR1##
In the protecting group elimination reaction in this case, tert-butoxycarbonyl-protected polyhydroxystyrene, (CH.sub.2 CH).sub.y (C.sub.6 H.sub.4 OH)(CH.sub.2 CH).sub.1-y {C.sub.6 H.sub.4 OC(O)OC(CH.sub.3).sub.3 }, and a hydrogen ion generated from the photochemical acid generator react with each other to generate polyhydroxystyrene, isobutylene (C(CH.sub.3).sub.2 CH.sub.2), carbon dioxide, and a hydrogen ion as illustrated in Chemical Formula 2 below. On this occasion, since isobutylene, the product of the protecting group elimination reaction, has a low boiling point (-6.9.degree. C.), it is evaporated, and does not remain in the photoresist. Therefore, no reverse reaction of the protecting group elimination reaction occurs. ##STR2##
However, the chemical amplification photoresist comprising the tert-butoxycarbonyl-protected resin has the defect that the shape of the resist pattern tends to be deteriorated if heat treatment is not carried out immediately after the exposure, i.e., it has a poor standing stability. In other words, it has the defect that the photoresist without heat treatment immediately after the exposure absorbs ammonia in the atmosphere and the ammonia causes a neutralization reaction with the hydrogen ion existing in the photoresist and heat treatment thereafter will not allow the elimination of protecting groups so that the resist remains insoluble in a developer, which results in the deterioration of the resist pattern.
On the contrary, Hattori et al. found that use of an acetal-based chemical group such as a tetrahydropyranyl group as a protecting group increases the standing stability of the resist.
On this occasion, in the protecting group elimination reaction, tetrahydropyranyl-protected polyhydroxystyrene, (CH.sub.2 CH).sub.y (CH.sub.6 H.sub.4 OH).sub.1-y (C.sub.6 H.sub.4 OC.sub.5 H.sub.4 O), and hydrogen ion generated by the photochemical acid generator upon irradiation with far ultraviolet rays react with each other to produce polyhydroxystyrene and tetrahydropyran (intermediate), (C.sub.5 H.sub.4 O).sup.+ as illustrated in the following Chemical Formula 3. ##STR3##
However, although the photoresist comprising the conventional tetrahydropyranyl-protected resin has the advantage that it is excellent in standing stability, it has the defect that it has a low dissolution contrast (in other words, the ratio of the dissolution speed of exposed portions to non-exposed portions of the photoresist) so that the photoresist has poor resolution. This is because tetrahydropyran (an intermediate) produced during the protecting group elimination reaction does not evaporate (boiling point: 88.degree. C.) and remains in the photoresist to thereby allow a reverse reaction to proceed in which tetrahydropyran reacts with polyhydroxystyrene, so that the protecting group elimination reaction in this case is an equilibrium reaction.
FIG. 1 is a graph illustrating the PEB time dependence of protecting group content in a conventional chemical amplification photoresist. As illustrated in FIG. 1, it can be seen from the variation of the protecting group content with the passage of the PEB time that the tetrahydropyranyl group gives earlier saturation than the tert-butoxycarbonyl group (t-BOC group) so that the protecting group elimination reaction does not proceed sufficiently in the chemical amplification photoresist having a tetrahydropyranyl group.